1. Field of the Invention
The present invention relates to an electrochemical cell and a process for splitting a solution and producing a hydroxide, sulfuric acid and a halogen gas. In particular, the cell and the process may convert either an anhydrous or an aqueous hydrogen halide, such as hydrogen chloride, to a halogen gas, such as chlorine gas, and a solution such as a sodium sulfate aqueous solution, to form sulfuric acid and sodium hydroxide, along with its co-product, hydrogen.
2. Description of the Related Art
Hydrogen chloride (HCl) or hydrochloric acid is a reaction by-product of many manufacturing processes which use chlorine. For example, chlorine is used to manufacture polyvinyl chloride, isocyanates, and chlorinated hydrocarbons/fluorinated hydrocarbons, with hydrogen chloride as a by-product of these processes. Because supply so exceeds demand, hydrogen chloride or the acid produced often cannot be sold or used, even after careful purification. Shipment over long distances is not economically feasible. Discharge of the acid or chloride ions into waste water streams is environmentally unsound. Recovery and feedback of the chlorine to the manufacturing process is the most desirable route for handling the HCl by-product.
A number of commercial processes have been developed to convert HCl into usable chlorine gas. See, e.g., F. R. Minz, "HCl-Electrolysis--Technology for Recycling Chlorine", Bayer AG, Conference on Electrochemical Processing, Innovation & Progress, Glasgow, Scotland, UK, Apr. 21-23, 1993.
Currently, thermal catalytic oxidation processes exist for converting anhydrous HCl and aqueous HCl into chlorine. Commercial processes, known as the "Shell-Chlor", the "Kel-Chlor" and the "MT-Chlor" processes, are based on the Deacon reaction. The original Deacon reaction as developed in the 1870's made use of a fluidized bed containing a copper chloride salt which acted as the catalyst. The Deacon reaction is generally expressed as follows: ##STR1## where the following catalysts may be used, depending on the reaction or process in which equation (1) is used.
______________________________________ Catalyst Reaction or Process ______________________________________ Cu Deacon Cu, Rare Earth, Alkali Shell-Chlor NO.sub.2, NOHSO.sub.4 Kel-Chlor Cr.sub.m O.sub.n MT-Chlor ______________________________________
The commercial improvements to the Deacon reaction have used other catalysts in addition to or in place of the copper used in the Deacon reaction, such as rare earth compounds, various forms of nitrogen oxide, and chromium oxide, in order to improve the rate of conversion, to reduce the energy input and to reduce the corrosive effects on the processing equipment produced by harsh chemical reaction conditions. However, in general, these thermal catalytic oxidation processes are complicated because they require separating the different reaction components in order to achieve product purity. They also involve the production of highly corrosive intermediates, which necessitates expensive construction materials for the reaction systems. Moreover, these thermal catalytic oxidation processes are operated at elevated temperatures of 250.degree. C. and above.
Electrochemical processes exist for converting aqueous HCl to chlorine gas by passage of direct electrical current through the solution. The current electrochemical commercial process is known as the Uhde process. In the Uhde process, aqueous HCl solution of approximately 22% is fed at 65.degree. to 80.degree. C. to both compartments of an electrochemical cell, where exposure to a direct current in the cell results in an electrochemical reaction and a decrease in HCl concentration to 17% with the production of chlorine gas and hydrogen gas. A polymeric separator divides the two compartments. The process requires recycling of dilute (17%) HCl solution produced during the electrolysis step and regenerating an HCl solution of 22% for feed to the electrochemical cell. The overall reaction of the Uhde process is expressed by the equation: ##STR2## As is apparent from equation (2), the chlorine gas produced by the Uhde process is wet, usually containing about 1% to 2% water. This wet chlorine gas must then be further processed to produce a dry, usable gas. If the concentration of HCl in the water becomes too low, it is possible for oxygen to be generated from the water present in the Uhde process. This possible side reaction of the Uhde process due to the presence of water, is expressed by the equation: EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e.sup.- ( 3)
Further, the presence of water in the Uhde system limits the current densities at which the cells can perform to less than 500 amps/ft..sup.2, because of this side reaction. The side reaction results in reduced electrical efficiency and corrosion of the cell components.
Another electrochemical process for processing aqueous HCl has been described in U.S. Pat. No. 4,311,568 to Balko. Balko employs an electrolytic cell having a solid polymer electrolyte membrane. Hydrogen chloride, in the form of hydrogen ions and chloride ions in aqueous solution, is introduced into an electrolytic cell. The solid polymer electrolyte membrane is bonded to the anode to permit transport from the anode surface into the membrane. In Balko, controlling and minimizing the oxygen evolution side reaction is an important consideration. Evolution of oxygen decreases cell efficiency and leads to rapid corrosion of components of the cell. The design and configuration of the anode pore size and electrode thickness employed by Balko maximizes transport of the chloride ions. This results in effective chlorine evolution while minimizing the evolution of oxygen, since oxygen evolution tends to increase under conditions of chloride ion depletion near the anode surface. In Balko, although oxygen evolution may be minimized, it is not eliminated. As can be seen from FIGS. 3 to 5 of Balko, as the overall current density is increased, the rate of oxygen evolution increases, as evidenced by the increase in the concentration of oxygen found in the chlorine produced. Balko can run at higher current densities, but is limited by the deleterious effects of oxygen evolution. If the Balko cell were to be run at high current densities, the anode would be destroyed.
Electrodialysis, a technology based on the use of cation-exchange and/or anion exchange membranes and of an electric field to effect ion separations, has been the subject of many recent books, articles and papers. See, for example, "Membrane Handbook", edited by W. S. W. Ho and K. K. Sirkar, Van Nostrand Reinhold, New York; and "The Green Potential of Electrochemistry, Part 2 Application" by D. Pletcher and N. L. Weinberg, Chemical Engineering, November, 1992, pg. 132-141. In recent years, DeNora Permelec, SpA, of Milan, Italy, has developed the HYDRINA.RTM. process, which generates sodium hydroxide without producing chlorine, using principles of electrodialysis. See "HYDRINA.RTM. Membrane Electrolyzers", published by DeNora Permelec, SpA. In this process, hydrogen and aqueous sodium sulfate are typically fed to a cell, where sodium hydroxide and hydrogen are formed. Commercial applications of electrodialysis now exist in brackish water desalination, treatment of industrial effluents, and in many other areas. See Ho and Sirkar, "Industrial Applications of Electrodialysis and Related Processes", Table 20-1.
A very recent review of salt splitting utilizing electrodialysis was given at the "Conference on Electrochemical Processing, Innovations and Progress", April 1993, Glasgow, Scotland, UK, entitled "Novel Approaches to Salt Splitting" by D. Genders, D. Hartsough and J. Thompson, pp. 21-23. FIG. 1 herein, which is described in this paper, shows a schematic of a three-component cell, shown generally at 1, for splitting sodium sulfate into sulfuric acid and sodium hydroxide. As shown in FIG. 1, cell 1 includes an anode 2, an anion-exchange membrane membrane 4, a cation exchange membrane 6 and a cathode 8. An aqueous solution of sodium sulfate is introduced into a central compartment bounded by anion exchange membrane 4, facing anode 2. Under the influence of an impressed electric field, sulfate ions migrate toward the anode and pass through the anion exchange membrane, where they combine with protons produced from water. The water is introduced into the anode compartment, where, by means of an electrochemical reaction, it is converted into protons and oxygen. Sodium ions from the sodium sulfate migrate through cation-exchange membrane 6 to the cathode compartment, which is also fed with water or dilute sodium hydroxide, whereby they combine with hydroxyl ions formed by the electrochemical reaction of the reduction of water to hydrogen and hydroxyl ion. The products of the electrodialysis are sulfuric acid and sodium hydroxide, and the gases, oxygen and hydrogen. The key to this electrodialysis process is the use of ion-selective membranes, that is, membranes which permit only cations (cation exchange membranes) or anions (anion exchange membranes) to pass therethrough, and reject ions of opposite charge. A number of such membranes are now commercially available, such as a membrane which is made of hydrated, copolymers of polytetrafluoroethylene and poly-sulfonyl fluoride vinyl ether-containing pendant sulfonic acid groups, and sold under the trademark "NAFION.RTM." (hereinafter referred to as NAFION.RTM.) by E. I. du Pont de Nemours and Company of Wilmington, Delaware (hereinafter referred to as "DuPont").
The difficulty of disposing of, which includes selling, hydrogen chloride, has been discussed above. In addition, the problem of disposing of large quantities of salts is becoming an increasing concern, in particular, the disposal of sodium salts, especially the sulfate which is formed as a by-product of pulp and paper industries, rayon plants, acid waste neutralization, pharmaceutical processes and other kinds of chemical process. See "HYDRINA.RTM. Membrane Electrolyzers", supra. In order to dispose of hydrogen chloride and sodium salts, several distinct, separate processes have been necessary. Thus, the need exists for a single process and apparatus for an environmentally and economically attractive technique to recycle these materials into useful products, which in many cases, may be key starting materials for other processes.